1. Technical Field
The present invention relates to a surface acoustic wave (SAW) filter having ground terminals separated, in which the ground terminals connected to inter-digital transducer (IDT) electrodes are separated in order to improve the attenuation effect of the frequency of an input electrical signal in the SAW filter in which the IDT electrodes are disposed in the propagation direction of a SAW. Furthermore, the present invention relates to a double mode coupled (DMS) SAW filter having ground terminals separated, which have the same pass band as an existing SAW filter, but can improve a skirt characteristic by increasing the attenuation effect of the DMS SAW filter in such a manner that the configuration of the ground terminals is changed by separating the ground terminals disposed in a SAW filter unit to which a DMS filter unit has been connected in series.
2. Description of the Related Art
In general, a SAW filter is a device produced by applying the characteristics of a SAW, transmitted on a surface of a substrate, as a frequency selection function device. In the SAW filter, a converter of a comb pattern, that is, a metal electrode, is formed in a substrate having a piezo-electric property. When an electric field is applied to the substrate, a surface of the substrate is temporarily twisted. A SAW is generated by such an action. The SAW generated as described above delays an input electrical signal or transmits only a specific frequency signal because it is slower than electromagnetic waves.
In this case, the metal electrode formed on the substrate having a piezo-electric property is called an IDT electrode. The IDT electrode functions to apply an electrical signal to a piezo-electric body and to convert a SAW generated from the piezo-electric body into an electrical signal. Such an IDT electrode is formed by alternately crossing a plurality of electrode fingers in a comb pattern.
The SAW filter includes various types. In particular, a ladder type SAW filter having an excellent skirt characteristic on the high frequency side is chiefly used in the transmission stage of a mobile communication terminal. A DMS SAW filter having an excellent skirt characteristic on the low frequency side of a pass band is chiefly used in the reception stage of a mobile communication terminal.
If the size of such a conventional SAW filter is reduced, it is difficult to obtain an attenuation effect. Accordingly, there is a problem in that a skirt characteristic on the high frequency side of a transmission stage or a skirt characteristic on the low frequency of a reception stage is deteriorated.